Sheet titanium, a moulded element produced therefrom and a method for producing the sheet titanium and the moulded element

ABSTRACT

The invention relates to a roll-bonded titanium sheet ( 6 ), a shaped component manufactured therefrom ( 10 ) and a method for manufacturing the titanium sheet ( 6 ) and the shaped component ( 10 ). In order to achieve a high-temperature-resistant shaped component ( 10 ), a titanium sheet ( 2 ) is roll-bonded at least on one side with aluminium foil ( 4 ) whose thickness (d) is small compared with the thickness (D) of the titanium sheet ( 2 ). As a result of heat treatment of the roll-bonded titanium sheet ( 6 ), the aluminium and titanium from the adjoining region are converted to an aluminium-titanium alloy. The outer titanium-aluminium-alloy layer of the titanium sheet ( 6 ) thus formed is converted by contact with oxygen into a titanium-aluminium-mixed oxide layer which gives the titanium sheet ( 6 ) good corrosion protection. The forming of the shaped component ( 10 ) preferably takes place before the heat treatment for alloy formation because the roll-bonded titanium sheet ( 6 ) is then still slightly deformable.

[0001] Since a sheet of a single metal generally does not have optimum properties (strength, corrosion protection, deformability, low weight) for specific operational purposes in every respect, composite materials have been developed, consisting of roll-bonded sheets of various materials. In particular, a roll-bonded sheet and its manufacture is known (U.S. Pat. No. 3,711,937 A) consisting of an aluminium sheet as the base sheet and a titanium cladding film. In order to join the two components together by roll bonding, after preheating to approximately 500° C. these are fed to a roll stand in which they are joined securely one to the other, associated with a thickness reduction of up to 50%. In order to improve the quality of the titanium-aluminium join at the boundary layer, heat after-treatment then takes place at temperatures up to approximately 600° C. A characteristic feature of such a composite sheet is that the aluminium sheet is covered with the film of pure titanium on its outer side. Such composite sheets may be well-suited for various applications such as aircraft building, heat exchangers and electrochemical plants. However, they are not suitable for high-temperature applications such as, for example, as components in exhaust gas systems of internal combustion engines where temperatures far above 600° C. are common, because the pure aluminium does not withstand the prevailing operating temperatures and the pure titanium does not offer sufficient corrosion protection.

[0002] The object of the invention is to develop a sheet and a method for its manufacture that can be used in the high-temperature range up to approximately 800° C., that offers sufficient corrosion protection and can be formed into a shaped component.

[0003] This object is solved using a titanium sheet in panel and strip format for operational use in the high-temperature range, which is roll-bonded at least on one side with an aluminium foil, whose thickness is small compared with the thickness of the titanium sheet.

[0004] Such a titanium sheet is manufactured in the following procedural steps:

[0005] a) The titanium sheet is roll-bonded with an aluminium foil at least on one side.

[0006] b) The aluminium and the titanium from the adjoining region are converted into a titanium-aluminium alloy by heat treatment of the roll-bonded titanium sheet.

[0007] c) The outer titanium-aluminium alloy layer of the titanium sheet is converted into a titanium-aluminium mixed oxide layer by contact with oxygen.

[0008] The titanium-aluminium alloy and mixed oxide layer formed only when the titanium sheet is used in operation as authorised gives the titanium a substantially better corrosion protection than titanium direct from the works. The disadvantage that the titanium sheet is difficult to shape after the alloy formation and just after the formation of the titanium-aluminium-mixed oxide layer can be avoided if the titanium sheet is formed into the shaped component before the heat treatment. It is especially advantageous if the heat treatment takes place only during the operational use of the shaped component as authorised, for example, by means of the heat prevailing in the exhaust gas system of internal combustion engines.

[0009] In order that sufficient corrosion protection is obtained by heat treatment of the titanium sheet which is still justifiable in terms of manufacturing technology, the aluminium foil should be very thin compared with the titanium sheet. Good experience has been obtained with a thickness ratio of the aluminium foil used and the titanium sheet used being in the range of a power of ten, especially with a titanium sheet having a thickness between 1 and 2.5 mm and an aluminium foil having a thickness between 0.01 and 0.2 mm. After the roll bonding associated with a reduction in the thickness of the aluminium foil, the aluminium layer should be between 0.02 and 0.06 mm. Such thin aluminium layers can be alloyed through their full thickness without any problem so that after the formation of the mixed oxide the titanium sheet has optimum, permanent corrosion protection even under extreme operating conditions such as those prevailing in the exhaust gas system of a combustion engine.

[0010] The invention is explained in greater detail in the following with reference to a schematic drawing showing the method according to the invention. Aluminium foil 2 having a thickness of 0.01 to 0.2 mm is drawn from a coil 1 and titanium sheet 4 in strip form having a thickness of 1 to 2.5 mm is drawn from a coil 3 and fed to a roll stand 5 at ambient temperature. As a result of the rolling pressure and the friction of the aluminium foil 2 and the strip-like titanium sheet 4 one above the other, an increase in temperature occurs in the rolling gap which is sufficient to bond the aluminium foil 2 onto the strip-like titanium sheet 4. Any oxide layer which may adversely affect the join is sufficiently broken up by the friction between the aluminium foil 2 and the strip-like titanium sheet 4. After leaving the roll stand 5, the aluminium-clad titanium strip 6 is fed to shears 7 which cut it to length into panels 8. The panels 8 can be stacked or supplied for further processing 9, for example, to an installation for cutting and forming into shaped components 10. Since no alloy formation between the strip-like titanium sheet 4 and the aluminium foil 2 has taken place because of the lack of heat treatment up till this time, the aluminium-clad titanium strip 6 can easily be deformed as composite sheet. Finally, the shaped components 10 are delivered to heat treatment 11 which allows the alloy profile to be adjusted by temperature and time control. The shaped components 10 with the alloy layer on one side are then exposed to the oxygen atmosphere so that the desired titanium-aluminium-mixed oxide layer forms as a corrosion protective layer.

[0011] It is to be understood that an aluminium foil can be bonded onto the titanium sheet 4 not only on one side, as in the exemplary embodiment, but also on both sides. In this case, a further coil of aluminium foil is then provided.

[0012] The invention is especially good for dividing the work. The manufacturer of the aluminium-clad titanium sheet either supplies this in panel format or as a coil to a processor which: then produces blanks and shaped components 10 therefrom. The processor can carry out the heat treatment of the shaped components 10 for alloy formation as the final production stage but this can be dispensed with if the shaped components 10 are exposed during subsequent operational use to operating temperatures sufficient for alloy formation.

[0013] To sum up, the following advantages are obtained as a result of the invention: light weight and high strength as a result of the material used, i.e., titanium, good corrosion protection as a result of the titanium-aluminium-mixed oxide layer, easy deformability of the titanium sheet before the alloy formation of the titanium-aluminium-mixed oxide layer and strength at high temperatures (far above 600° C.). 

1. A titanium sheet in panel or strip format for operational use in the high-temperature range that is roll-bonded at least on one side with an aluminium foil (4) whose thickness is small compared with the thickness of the titanium sheet (2).
 2. A shaped component (10) made of titanium sheet (6) according to claim
 1. 3. A use of a shaped component (10) as a component in an exhaust gas system of a combustion engine.
 4. A method for manufacturing a roll-bonded titanium sheet (6) according to claim 1 or 2, characterised by the following procedural steps: a) The titanium sheet (2) is roll-bonded with an aluminium foil (4) at least on one side. b) The aluminium and titanium from the adjoining region are converted to a titanium-aluminium alloy by heat-treatment of the roll-bonded titanium sheet. c) The outer titanium-aluminium alloy layer of the titanium sheet (6) i s converted into a titanium-aluminium-mixed oxide layer by contact with oxygen.
 5. The method according to claim 4, characterised in that the thickness ratio (d/D) of the aluminium foil used (4) and the titanium sheet used (2) is in the range of a power of ten.
 6. The method according to claim 4 or 5, characterised in that the thickness (D) of the titanium sheet (2) used is between 1 and 2.5 mm and the thickness (d) of the aluminium foil used (4) is between 0.1 and 0.2 mm.
 7. The method according to any one of claims 4 to 6, characterised in that the thickness of the aluminium layer after the roll-bonding is between 0.02 and 0.06 mm.
 8. The method according to any one of claims 4 to 7 for manufacturing a shaped component (10) according to claim 2, characterised in that the forming of the roll-bonded titanium sheet (6) into the shaped component (10) takes place before the heat treatment for alloy formation.
 9. The method according to any one of claims 4 to 8 characterised in that the heat treatment for alloy formation only takes place during the operational use of the shaped component (10) as authorised. 